This invention relates to infrared sensors which may be used for automatic light switching or for intruder detection by sensing the thermal infrared radiation emitted by a human being in the vicinity of the sensor. In particular it relates to an infrared sensor comprising an optical system for gathering and concentrating infrared radiation from a source and a pyroelectric radiation detector for receiving the infrared radiation and generating an output signal. Such sensors usually comprise an array of lenses for directing and concentrating radiation from a plurality of arcuately displaced directions onto a detector.
In European Patent Application No. 0,197,583 A1 which corresponds to U.S. Pat. No. 4,717,821 an array of fresnel lenses, moulded in planar form, is described for use in an infrared intruder alarm. The passage of an intruder across any one of the arcuately displaced directions generates a signal in a detector placed to receive the image of the intruder focused by the associated lens. Each direction is defined by the line joining the detector to the pole of the associated lens. The angular width of the radiation sensitive zone associated with each of the directions is defined by the ratio of the detector width seen from that direction to the focal length of the associated lens. To match likely source outlines for effective sensing the detector shape and dimensions are required to bear a definite relationship to lens focal length. With a typical detector area of 2 mm.times.1 mm, a focal length of 30 mm would be optimum.